Fix detection of cyclic structs and unions

[kornilova203]

Closes #103
Closes #115

Removed CycleDetection because it supported only limited cases and also it could find a cycle in a unsuccessful place: in a record that uses value instead of pointer (cycles may be "broken" only in pointer fields).

So types in IR can contain cycles.
usesType method was modified such that it does not get stuck in cycles.

To do:

• Break cycle only in one place
• Add tests for unions and more complex tests for structs
• Support more complex cases like fields of function pointer type

[kornilova203]

I'll review it after #114 is merged

[kornilova203]

To break a cycle in one place:

1. find all structs and unions in the cycle
2. filter out records that use value type
3. Decide where cycle will be broken. For example it can be the struct/union that is defined after all other structs/unions or struct/union name of which is alphabetically largest/lowest.

It is also a good idea to minimize amount of fields where a cycle is broken (this is relevant for situations when one or more fields belong to multiple cycles).
It looks like a problem of finding minimum dominating set:

1. create a node for each cycle and for each cyclic field
2. connect each node of cyclic field to nodes of corresponding cycles.
3. connect all nodes of cyclic fields to each other.
4. find a minimum dominating set.
5. cut cycles in node that belong to mds.
   (it is always possible to find minimum dominating set that consist only of cyclic fields nodes)

Maybe it can be done by simply picking fields that belong to maximum number of cycles, but I cannot prove that now.
Anyway, this situation is pretty rare, so I am not going to solve that now 😄

[kornilova203]

Just for completeness:
If we want to minimize number of "cuts" that brake cycles it should not be done by "cutting" fields that are shared by maximum number of cycles. Consider this situation:

According to greedy algorithm we would "cut" vertex B and then A and C but optimal solution is to "cut" only A and C.

[kornilova203]

All supported cases are listed in tests/samples/Cycles.h
I think that this commit covers all possible cases but maybe I am not aware of some of them